But I have problems that this macro does not produce 0mm spacing in a table cell (just at the beginning of the cell!) if I call zero times \multspc{0}.
There is always some indent, but where does this come from?

I need this macro to indent some algorithm in the table cell, its nasty but manualy formating gives me the best result in a longtable, because its multiple pages long...

The problem with the current macro is that it unwanted whitespace is added from due to indentation. Add some % add the line ends to get rid of these. However, the current definition also produces a warning after you use \mulspc more than once. That is because the counter definition \newcounter{ctra} is contained within the \newcommand{\mulspc}. Rather define the counter outside the macro definition. Another drawback is that the space offered by \mulspc{0} should be non-existent, which it is not, even if you add % to remove the unwanted spaces. For example, the code: